1. Field of the Invention
This invention relates to the use of ring lasers and in particular to the use of a mirror to suppress one of the two outputs typically produced by a ring laser.
2. Description of the Prior Art
As shown in FIG. 1, a ring laser typically emits light in two separate directions. Since some applications of these lasers require only one output, the other output must be suppressed. The unwanted output has been named the "reverse wave", and the desired output the "forward wave".
Two techniques have evolved to accomplish reverse wave suppression. One involves polarizing elements inside the laser cavity, and the other uses a mirror outside the laser cavity to direct one of the outputs back into the cavity in the opposite direction. The former technique is limited to ring lasers of relatively low power, because polarizing elements cannot survive high light intensities. The reverse wave suppressor mirror technique can be used for all types of ring lasers. The use of mirrors is particularly desirable because the mirror can reinject the reverse wave in the forward direction, thus reinforcing the forward wave.
Conventional reverse wave suppressor mirrors are designed to match the reverse wave radius of curvature to within a fraction of the wavelength of the laser light. Smooth, high tolerance mirrors are selected for this purpose.
Conventional reverse wave suppressor mirrors are manufactured to surface tolerances of tens of nanometers, as are all of the other mirrors in the laser cavity. The surface profile of a conventional reverse wave suppressor mirror is shown in FIG. 2a. Scratches on the surface of a conventional mirror may be from 0.5 to 2 nanometers deep and separated by 0.1 to 0.5 millimeters.
There are at least two significant shortcomings in using conventional mirrors to suppress the reverse wave. The first shortcoming is that conventional mirrors are extremely sensitive to angular misalignment. Misalignment can be caused by tilting the mirror, by undesirable aberrations on the mirror surface or by variations in the optical bench structure supporting the mirrors.
Misalignment can significantly reduce resonator performance. For example, the figure of merit for the performance of the ring laser is known as the far-field brightness, P.sub.F /n.sup.2. That figure is the fraction of the total laser power contained in the central spot of the focused beam. The experimental data displayed in FIG. 3 shows a factor of two decrease in far-field brightness with 150 microradian tilt of the conventional suppressor mirror. Misalignment can also generate significant excess heat, thereby increasing cooling requirements.
The second significant shortcoming in using conventional mirrors to suppress the reverse wave is the complexity of manufacturing and mounting such mirrors. As shown in FIG. 3, conventional mirrors rapidly lose effectiveness when tilted, and tilt can be caused by movement of the mirror.
There are certain manufacturing and mounting techniques that can be used to reduce the movement of the reverse wave suppressor mirror. For example, a corner cube retroreflector can be used as a reverse wave suppressor mirror to reduce sensitivity of the mirror to tilt. However, since the corner cube involves three reflections and displaces all three pieces of the reverse wave upon retroreflection, it can only be used on ring lasers whose reverse output has three-fold symmetry. Furthermore, corner cubes consist of three precisely-aligned, aberration-free conventional mirrors, and their construction exceeds the cost and complexity of conventional reverse wave suppressor mirror systems.
It is therefore an object of this invention to provide a reverse wave suppressor mirror system which reduces the sensitivity of the system to tilt.
It is also an object of this invention to significantly reduce the cost and complexity of the manufacture and mounting of reverse wave suppressor mirror systems.